Electronic resonators are used in a variety of electronic circuits to perform a variety of functions. Depending upon the structure and material of the resonator, when an AC signal is applied to the resonator over a broad frequency range the resonator will resonate at specific resonant frequencies. This characteristic allows the resonator to be used, for example, in an electronic filter that is designed to pass only frequencies in a preselected frequency range, or to attenuate specific frequencies. Many applications would be ideally served by resonators and filters which are electrically tunable, thus minimizing the added noise and interference associated with their wider bandwidth fixed tuned counterparts.
Resonators are also used in high frequency applications, such as optical and wireless communication systems which operate in the GHz range. In these types of applications, resonators are used, for example, to stabilize the frequency of oscillators in transmitters and receivers. These types of resonators must exhibit high Q values in order to provide the necessary oscillator frequency stability and spectral purity, and also maintain low phase noise. Many oscillators used in communications systems employ a Voltage Controlled Oscillator (VCO), which is electronically tuned to an exact frequency or set of exact frequencies (or channels), by means of a voltage variable reactance (typically a varactor diode) coupled to a fixed frequency resonator. A control voltage applied to the voltage variable reactance tunes the resonant frequency of the resonator, and consequently tunes the oscillator frequency. This voltage tunability of frequency enables compensation for the effects of manufacturing tolerance, temperature, aging and other environmental factors affecting the frequency of oscillation. At microwave frequencies, gallium arsenide varactor diodes are normally employed in this application because these have a relatively high Q. Their Q, however, is typically less than 50 at 10 GHz, which is still low compared to the available Q of fixed frequency resonators. As a result, the performance of oscillators and filters utilizing electronic tuning tend to exhibit higher noise and losses compared to their fixed frequency counterparts.
While several types of high Q fixed frequency resonators known in the art can be used in high Q applications, including, for example, cavity resonators, coaxial resonators, transmission line resonators and dielectric resonators, voltage tunable high Q resonators have not heretofore been known. In view of the above, it would be desirable to provide a voltage tunable high Q resonator that can be designed to resonate at a variety of specific resonant frequencies while having a simple structure and which is inexpensive to mass produce using proven materials (e.g., ceramics) and proven microelectronic techniques (e.g., lithography).